Programmable, servo-controlled machines like industrial robots are often equipped with position switches for limiting their workspace or allow interlocking of the machine with other moving equipment. These position switches often use a machined cam to define the switching positions. In particular positions switches are often used in applications where occupational safety regulations have to be followed.
U.S. Pat. No. 5,558,196 to Nehei et al entitled Device for detecting limits of rotational motion in a robot (Fanuc) a device for detecting the limits of rotational motion of a rotating member of an industrial robot, wherein the rotating member rotates in a first and a second rotational direction about an axis relative a fixed member. The device comprises a limit switch provided on a rotating member and a contact piece, provided on the fixed member to activate the limit switch. The device does not appear to require adjustment of the contact piece. However the casting for the arm appears to require additional material to form a circular wall around the arm followed by machining to shape a circumferential wall on the rotating member to which the contact piece will be attached.
Generally speaking, mechanical position switches are expensive, require maintenance, and need additional cables between machine and controller. Each limit switch generally requires periodic replacement and/or repair.
Solutions are known that use an electronic supervision of reference and measured position of at least one servo axis providing a position switch function that does not rely on limit switches and mechanical cams. U.S. Pat. No. 6,778,867 to Ziegler et al, entitled Monitoring and control of a handling device, describes a monitoring and control device for monitoring a technical system having at least one portable and/or mobile and/or immobile device, and more specifically, a handling device that is a arranged in a protective device, and further including at least one preferably central or decentralized control unit and actuators connected thereto to carry out dangerous actions.
Electronic supervision solutions may typically require that a difference between a reference value and a measured position shall stay below a predetermined threshold. This type of supervision may however, trigger a stop because the control error exceeds the threshold due to some unforeseen robot movement. Amongst other things, a difference between reference and measured positions, which may be seen as a type of servo control error, may increase due to dynamic effects such as servo lag and transients during acceleration. As a consequence, the threshold may be set at a level which less than optimal.